Tungsten oxide materials are useful for their electrochemical and electrochromic behavior, and they are widely used in electrochromic devices. There are a number of different deposition techniques and polymorphs of tungsten oxide that have been used in electrochromic devices. Amorphous tungsten trioxide deposited by either thermal evaporation or sol-gel methods, is often used for electrochromic applications although crystalline tungsten trioxide is also used.
Commercial switchable glazing devices, also commonly known as smart windows and electrochromic (EC) window devices, are well known for use as mirrors in motor vehicles, aircraft window assemblies, sunroofs, skylights, and architectural windows. Such devices may comprise, for example, active inorganic electrochromic layers, organic electrochromic layers, inorganic ion-conducting layers, organic ion conducting layers and hybrids of these sandwiched between two conducting layers. When a voltage is applied across these conducting layers the optical properties of a layer or layers in between change. Such optical property changes typically include a modulation of the transmissivity of the visible portion or the solar sub-portion of the electromagnetic spectrum. For convenience, the two optical states will be referred to as a bleached state and a darkened state in the present disclosure, but it should be understood that these are merely examples and relative terms (i.e., a first one of the two states is more transmissive or “more bleached” than the other state and the other of the two states is less transmissive or “more darkened” than the first state) and that there could be a set of bleached and darkened states between the most transmissive state and the least transmissive state that are attainable for a specific electrochromic device; for example, it is feasible to switch between intermediate bleached and darkened states in such a set.
The broad adoption of electrochromic window devices in the construction and automotive industries will require a ready supply of low cost, aesthetically appealing, durable products in large area formats. Electrochromic window devices based on metal oxides represent the most promising technology for these needs. Typically, such devices comprise two electrochromic materials (a cathode and an anode) separated by an ion-conducting film and sandwiched between two transparent conducting oxide (TCO) layers. In operation, a voltage is applied across the device that causes current to flow in the external circuit, oxidation and reduction of the electrode materials and, to maintain charge balance, mobile cations to enter or leave the electrodes. This facile electrochemical process causes the window to reversibly change from a more bleached (e.g., a relatively greater optical transmissivity) to a more darkened state (e.g., a relatively lesser optical transmissivity).
Electrochromic devices may utilize a combination of two types of electrochromic materials, one of which becomes optically less transmissive (e.g., takes on color) in its electrochemically oxidized state while the other becomes optically less transmissive (e.g., takes on color) in its electrochemically reduced state. Such a device where both anodic and cathodic electrochromic materials can simultaneously darken or bleach may be called a complementary electrochromic device. For example, Prussian blue assumes a blue color in its electrochemically oxidized state and becomes colorless by reduction while tungsten trioxide (i.e., WO3), assumes a blue color in its electrochemically reduced state and becomes colorless by oxidation. When the two are used as separate electrochromic layers separated by an ion conductor layer in a multi-layer stack, the stack may be reversibly cycled between a blue color (when the Prussian blue material is in its electrochemically oxidized state and tungsten trioxide is in its reduced state) and a transparent state (when the Prussian blue material is in its electrochemically reduced state and tungsten trioxide is in its electrochemically oxidized state) by application of an appropriate voltage across the stack.
For convenience of description herein, change of these one or more optical properties of electrochromic devices (i.e., switching or cycling of the electrochromic devices) is primarily discussed as occurring between a pair of optical states (i.e., an optically less transmissive state and an optically more transmissive state), but it should be understood that these are merely examples and relative terms. For example, the optically less and more transmissive states can be a pair of optical states between a pair of more extreme optically less and more transmissive states that are attainable by a specific electrochromic device. Further, there could be any number of optical states between the optically less and more transmissive states.
Tungsten oxides are well-known electrochemically active materials. Uncertainty exists in the literature, however, regarding whether crystalline or amorphous materials are preferred. In addition, while tungsten trioxide (WO3) crystallizes in several polymorphs, there is no clear preference as to which polymorph is best, or whether demonstrable differences should be expected. Crystallinity, the degree of crystallinity, and the crystal system obtained varies with synthesis method, temperature, the use of additives and other considerations.
Some electrochromic systems in prior art literature have studied amorphous WO3 prepared by physical vapor deposition (PVD). Many examples in the PVD literature teach that crystalline WO3 is detrimental for EC performance. Crystalline WO3 in these studies tends to be deposited onto a hot substrate, or crystallized from an amorphous deposition, and is most often of monoclinic symmetry.
In addition to thermal evaporation, WO3 can be deposited by a number of other methods, such as sol-gel, electrodeposition and hydrothermal synthesis. Different deposition methods can be used to prepare WO3 films with different crystal structures. While not every polymorph may be prepared in a straightforward manner, methods for the synthesis of each are known.
WO3 may be prepared from sol-gel methods requiring expensive precursors and organic solvents. These reagents contain significant amounts of carbon from their (often) alkoxide ligands and this carbon must be removed later in the process, often using elevated temperatures. In addition, some WO3 film deposition methods require the use of binders and templating agents to facilitate the synthesis of different polymorphs of WO3 and/or impart robust mechanical properties to the deposited films. These binders and templating agents remain in the film after deposition diluting the active material in the film, and/or require complex processing conditions to remove.
The prior art has shown the deposition of crystalline WO3 films that require high temperature treatments of the films on the substrate. One deposition method is hot wire chemical vapor deposition to form WO3 nanoparticles followed by electrophoretic deposition of a film, and another deposition method was thermal evaporation. Both techniques used required a post-deposition anneal at 300-400° C. for 2 hours to form the intended crystalline films. These techniques requiring high temperature post-deposition annealing are not feasible for films formed on substrates that can be altered or melt at those temperatures.
Crystalline tungsten trioxide films can be deposited directly on a substrate using techniques such as thermal evaporation or sol-gel methods with annealing. Sol-gel methods require high temperatures after deposition to create crystalline tungsten trioxide films with the most desirable electrochromic properties. Crystalline hexagonal tungsten trioxide nanostructures (e.g. nanowires and/or nanoparticles) can be grown on tungsten trioxide seed layers as well. Physical vapor deposited (e.g., evaporated or sputtered) films are typically amorphous as-deposited unless the substrate is heated, and may also thus require thermal treatments to crystallize the films after deposition. The high temperatures required to crystallize an as-deposited amorphous film produced by physical vapor deposition to form crystalline tungsten trioxide are incompatible with substrates with low melting points (e.g. flexible polymer substrates). The high temperatures and environments required for post-deposition annealing also require more expensive equipment. Additionally, some processing temperatures required for the crystallization of certain tungsten trioxide polymorphs can reach as high as 900° C., which precludes the use of glass substrates if the material is deposited directly onto the substrate before the crystallization step. These techniques all have drawbacks in manufacturing compared to hydrothermal synthesis of crystalline tungsten trioxide particles, followed by size-reduction and coating, especially when targeting crystalline particles of a specific symmetry or structure.
One of the crystal phases of WO3 that has been studied by certain processing techniques is hexagonal WO3 (i.e., h-WO3). There are some examples of hexagonal WO3 (i.e. h-WO3) for battery (i.e. electrochemical) and EC applications in the prior art. Commonly utilized synthetic methods such as PVD and electrodeposition, however, are not always amenable to the preparation of single phase, crystalline h-WO3. Instead, h-WO3 has been produced via hydrothermal synthesis, growing nanostructures directly on substrates, and producing nanostructures in solution.
Other crystal phases of WO3 have other symmetries and may be described as triclinic WO3 or monoclinic WO3 or cubic WO3, as is appropriate based on their symmetries. In general, if the arrangement of the atoms in the crystal phase are similar but differ in symmetry, the crystal phase may be described strictly by its symmetry. In certain circumstances, however, the arrangement of the atoms in the crystal phase may be unique beyond simple distortions that alter the lattice symmetry. In such cases, the use of structure types in addition to a symmetry descriptor is useful
Tungsten oxide thus may be described as displaying a number of polymorphs. The term “polymorph” is here intended to comprise symmetry changes that largely maintain some or all of the same atomic connectivity and unique relationships of atoms that produce unique structural features. Sometimes, however, polymorph may describe an entirely different structure and atomic arrangement but with the same composition. Critically, the synthesis and/or thin film deposition method can impact the resulting polymorph. For instance, thermally evaporated tungsten oxide is typically amorphous especially if the substrate is not heated. The resulting films can be crystallized by post-deposition annealing (e.g., in air), however, the resulting crystal structure of the tungsten trioxide so produced is typically monoclinic perovskite. Monoclinic perovskite tungsten oxide is known to undergo phase transformations upon intercalation (e.g. with Li). [Nonstoichiometric Compounds; Ward, R.; Advances in Chemistry; Chapter 23, pp 246-253, American Chemical Society: Washington, D.C., 1963.] Examples of tungsten trioxide materials that typically occur in different structures are sol-gel prepared materials and commercially available nanostructured materials which typically have the “Perovskite Tungsten Bronze” [or PTB] structure. The PTB structure may be described as similar to ReO3 in which metal (M) ions (usually monovalent) are intercalated into interstitial spaces of the ReO3 structure resulting in MxReO3 and the perovskite structure type. Also, it is well known in the literature that thermally evaporated films that have subsequently been crystallized by annealing show worse durability than the amorphous tungsten trioxide thermally evaporated films in electrochromic devices.
Another polymorph of WO3 is the cubic pyrochlore. Sometimes the stoichiometry is represented with waters of hydration and sometimes with hydroxides. Sometimes the stoichiometry is represented with counter ions and sometimes the stoichiometry is doubled, e.g. [—]W2O6. For simplicity, the pyrochlore phase will be described here as part of the WO3 series and explained as a substituted WO3 when additional metals are present.
The film deposition approaches of many crystalline tungsten oxide polymorphs have limitations. For example, h-WO3 nanowires formed by hydrothermal synthesis directly on substrates required a 400° C. pretreatment step to form a WO3 seed layer. This pretreatment step is not compatible with many low temperature substrates. Additionally, these films had poor mechanical properties, such as low adhesion to the underlying substrate. Hexagonal-WO3 nanostructures have also been produced in solution, but have either not been deposited into films (i.e. characterized as a colloidal dispersion only), or were deposited in a composite film using conductive carbon and binders (to address improving the mechanical properties, such as adhesion).
What is therefore desired are crystalline tungsten oxide electrochromic materials capable of forming films and devices on a variety of substrates, and methods for producing the same. Furthermore, an electrochromic device with crystalline tungsten oxide and methods for producing the same, with improved device durability, is also desirable. It is within this context that the embodiments arise.
Corresponding reference characters indicate corresponding parts throughout the drawings. Additionally, relative thicknesses of the layers in the different figures do not represent the true relationship in dimensions. For example in FIG. 2, the substrates are typically much thicker than the other layers. Unless otherwise noted, the figures are drawn only to illustrate connection principles, not to give any dimensional information.